Tons of threads on this, with several detailed posts so you can see what FTers say about their likes/dislikes of each, and make a call.
A has a mni-gym with very decent equipment if you need a quick workout.
A has the old "Flagship Lounge" area which is a quiet, peaceful, no-one-is-ever-there oasis at DFW like none other.
A has the best showers by far (the "personal car wash" as I call them--you'll see why when you see them, all jets & hoses everywhere, underneath the huge "rain" head).
D has the best views, and the restaurant options in terminal D, IMHO, are generally better.
The staff are no better/worse at one than another, as they seem to rotate around them all. You can easily get from one to another via Skylink in minutes, if you're doing an AAdmirals Club tour.